The present invention relates to an optical information pickup apparatus.
In an optical video disc player, or compact audio disc player or the like a track composed of pits on a recording surface of the disc is illuminated with a light spot, and changes in the reflected light from the track are converted to an electrical signal from which various information such as image information can be obtained.
In such an optical information pickup apparatus, a so-called focus servo signal and tracking servo signal as well as a regenerative (recorded information bearing) signal are simultaneously detected using a photoelectric element. Ths focus servo signal is used for accurately converging the illuminating beam onto the recording surface of the disc, whereas the tracking servo signal is used for maintaining the alignment of the illuminating beam with the track.
Currently available systems for detecting the focus servo signal employed in conventional apparatuses include an astigmatism system, a wedge-prism application system, an critical-angle prism application system, and a double-circle system. The astigmatism system, the wedge-prism system and the critical-angle prism systems are disadvantageous with regard to cost since optical parts such as a cylindrical lens or a prism and the like are used. The double-circle system involves a difficulty in achieving precise servo control since the light of the regenerative signal tends to interfere with that of the focus servo signal as the illuminating beam traverses the recording track of the disc. Moreover, the conventional optical information pickup apparatus requires other optical parts, such as an optical grating and the like, which are costly.
It is therefore an object of the invention to provide an optical information pickup apparatus which is capable of detecting the servo signal with considerable accuracy and which is of low cost.